U.S. Pat. No. 5,551,686 to Sanchez et al. discloses a mailbox unit comprising a plurality of bins adapted to received printed substrates from a printer. The '686 patent further describes a sensor for detecting an almost-full bin condition and a bin-full condition in a bin. The sensor comprises two integral switches and an actuating arm. The end of the arm opposite the switches is provided with a rotatable ball for engaging substrates. The switch and the arm may be mounted to or under the bottom of an overlying tray. The '686 patent teaches in column 10, lines 40-46, "[t]he switch arm 12c end location * * * should preferably not be located near the sheet stack edge, i.e., be more centrally located in the bin, so as to avoid stack height sensing errors from edge curled or bent paper in the bins changing the true stack height * * * ." The apparatus disclosed in the '686 patent further includes an optical bin-empty sensor.
U.S. Pat. No. 5,033,731 to Looney discloses a stack height and sheet delivery detector system for use in a printer output bin. The detector system comprises an elongated actuating arm 22 pivotally connected to a frame about an axis which extends generally transverse to a printed sheet output path. The arm normally rests by gravitational force on the top sheet of a stack of printed sheets in the printer output bin. Printed sheets are sequentially fed into the bin for stacking by exit feed rollers along the printed sheet output path. As each sheet passes through the feed rollers, its leading edge pushes forward and lifts the pivotal arm upward to a sheet delivery detection position. In this position, an extension of the arm functions to actuate an optical sensor providing an output signal indicative of an incoming sheet. The arm then drops down to its normal position where it rests on top of the upper-most sheet. Once the arm "rest" position on the top of the sheet stack rises above a preset level, the same or another sensor is actuated by the extension on the arm indicating that a desired stacking level has been attained.
Another known sensor for detecting at least one printed substrate fill condition in an output bin comprises a main body pivotably mounted under the bottom of an overlying tray about an axis which is generally parallel to a path that printed substrates follow as they move into the bin. The main body is spaced from an edge of its corresponding bin. It is shaped like an isosceles triangle and is mounted such that its two side edges of equal length are positioned toward the entrance into the bin and toward the end of the bin opposite the bin entrance. When in its home position, the main body is positioned at an angle of about 45 degrees to vertical. As substrates are fed into the bin, they engage the first side edge of the main body causing it to pivot upward. It is believed that the main body, when moved a sufficient distance upward, actuates a corresponding sensor such that the sensor indicates a "full bin" condition.
None of the sensors described above are capable of detecting when a portion of an edge curled or bent sheet extends to or above a level of a substrate output path immediately adjacent a bin entrance. If a portion of a curled or bent sheet extends to or above a level of the substrate output path immediately adjacent the bin entrance, it may cause a substrate feed failure, e.g., a paper jam, as an incoming substrate may engage the curled or bent substrate. Accordingly, there is a need for an improved sheet sensing mechanism for use in a printer output bin wherein the sensing mechanism is capable of detecting when a portion of an edge curled or bent sheet extends to or above a level of a substrate output path immediately adjacent a bin entrance.